The invention relates to a method for flow measurement of small liquid volumes passing through a tube-like element, in which a gas bubble is introduced, whose flow time in the longitudinal direction of the tube-like element is measured between two predetermined points located at a distance from each other, the diameter of the gas bubble being selected to be approximately the same as the inside diameter of the tube-like element, and the flow rate being calculated from the flow time. The invention also relates to a flow rate measurement device for the application of the method.
The problem of measuring very small liquid flow volumes, especially between 1 and 500 .mu.l/h is difficult, particularly for medication dispensers. Therefore, a satisfactory functional control has not been possible for miniaturized devices. However, since patients can be endangered if functional problems in such devices are not identified, such as control is urgently needed. Operational problems can occur in the dispensing device itself, as by breakdowns of the structural elements or battery deficiencies, and also in the dosage path, due to blockage by drug floccules of the narrow feeding hose or tube, or to deposits on the drug discharge opening. The need for a reliable functional control is even greater in the case of implantable medication despensers.
A method and a device of the aforementioned type are described in U.S. Pat. No. 3,739,636. This shows a method for measuring flow rates, in which two detectors are spaced a predetermined distance from each other. A needle or a three-way stopcock connected with a gas reservoir is used to feed a gas bubble, which enters the tube-like element and is carried along by the flow current. As the gas bubble passes by the first detector, the timing begins. As soon as it reaches the second detector, the timing ends. A lamp, on the one hand, and a photocell, on the other, are used as detectors. This arrangement has the disadvantage that where the flow measurement apparatus must be of the smallest possible size, for example, in medical applications such as medication dispensers, a lamp and a photocell are too bulky and are therefore unsuitable. Similar methods and devices are also described in British patent publication No. 2,083,612 and in U.S. Pat. No. 3,621,715.
DE-PS 831 610 also deals with the measurement of small volumes of liquid. In this reference, a gas bubble is fed into a gauge tube to measure fuel consumption in a motor vehicle. The procedure is repeated at regular intervals. For measurement purposes, two shutoff valves separate the gauge tube from the liquid flow. A scale is used to determine how far the gas bubble travels inside the tube. This reference states that electrical contacts located face-to-face inside the tube can be used as gas bubble detectors. For any variation in the resistance of the circuit in which the contacts and a power source are located, a relay closes. This arrangement is disadvantageous because it requires a complicated manual procedure for evaluating the flow rate, and this is not practical for a medication dispenser. Furthermore, measurement requires an indicator scale which cannot be used in an implantable medication dispenser.
DE-OS No. 27 52 328 is also related to fuel consumption measurement. Here, ions are generated by an ionising first electrode. The ions are carried with the fuel flow and are recaptured at a second electrode located downstream. The flow time is calculated from the travel time of the ions between the two electrodes. This device is stated to be especially suitable for measuring high flow rates, because it relies on longitudinal displacement of the ions, i.e., displacement in the direction of the flow. During low flow rates or small liquid flows, the ions would again recombine, and a false measuring signal would result.
DE-OS No. 22 18 459 describes a method for measuring liquid flow in a hydraulic system. In particular, it deals with the measurement of the mass flow of a discontinuous phase or of the flow volume of a liquid mixture containing a conductive fluid. The electric conductivity is measured with two electrodes which are in contact with the liquid flow. This measurement produces a signal indicating a change in conductivity which is the result of a change in the component ratio of the liquid mixture. The mass flow is calculated for the flow time from the measured conductivity changes. A prerequisite for the measurement is that the changes in conductivity take place at a high speed. They therefore trace back to turbulence. High flow rates are a prerequisite in this measurement method also. In contrast, the invention herein relates to the measurement and detection of small amounts of liquid and low flow rates in a tube-like element with a small cross-section.
One object of the present invention is to provide a method and a device which are especially suitable for the detection of very small volumes of liquid flow, particularly for medication dispensers, where few and small structural elements are desirable.